An important lesson to be learned when teaching a dog to walk nicely on a leash is to maintain a loose leash because if one puts tension on the leash, the dog will respond by pulling, or pulling harder. A great way to teach your dog not to pull is to reinforce the idea that if they pull, progress in that direction ceases. Some common methods used in leash training are standing still or turning in another direction. Usually "non-aversive" methods use some variation of stopping movement in the desired direction and rewarding the dog for staying in the desired position.
But it is not just having a taunt leash that may cause your dog to pull! There are at least two other forms of tension to which your dog may respond by pulling. One of them, I realized long ago (with the help of other trainers) and the other I experienced more recently.
First, there is the tension in the handler. Even if there is no tension in the leash and that wonderful "J" shape exists between the handler and the dog, the person's tension can move through the leash and transfer from the handler to the dog. There are many types of tension that travel through the leash. Something as simple as having a death grip on the leash or as complicated as being preoccupied by everything left undone for the day, can cause your dog to pick up the pace and practically drag you through that daily walk (or in the case of a service animal, the whole day).
But recently, I discovered another kind of tension that affected how well my dogs maintained a loose leash . . . a collar that was adjusted too tightly. Treun recently went through another growth spurt and I had not noticed that this one had affected his neck size. While his collars were not overly tight, they were more snug than before. Once I adjusted them to give him a looser fit, I noticed that he tended not to pull as often. Now, he is still challenged by high distractions when walking on leash, but the change was noticeable!
So remember, tension - no matter what the source - will contribute to your dog's desire to pull. Keep the leash loose, stay relaxed, and keep those collars comfy!